Re: Bad behaviour on some geometric operations on degenerate polygons

On Tue, Oct 15, 2013 at 04:03:58PM +0200, Josef Grahn wrote:
> Hello.
>
>
> I encountered an unexpected behaviour when doing some geometric operations on
> polygons that consist of only one point (or more generally, when all points in
> the polygon are identical).
>
> Specifically, taking the center point of such a polygon results in an error
> message.
>
> == Code to reproduce ==
> SELECT @@ polygon '((1, 1))';
>
> == Expected result ==
> (1,1)
>
> == Actual result ==
> ERROR: cannot convert empty polygon to circle
>
> The same problem exists when one tries to convert a polygon to a point using
> the point() function (presumably because the same code path is invoked).
> Ironically, the consequence of this is that the only non-empty polygon that can
> *not* be converted to a point, is one that occupy precisely a single point.
>
> Other operations, such as overlap tests, on the other hand seem to work as one
> would expect, which leads me to conclude that handling of degenerate polygons
> is meant to function, and that the behaviour of this particular operation is
> simply a bug.
>
> This bug report applies to at least PostgreSQL versions 9.1.9 and 9.3.0. Other
> versions, I have not tested.

Thank you for the report;  I am just getting to study it.

I think you are right.  What is happening is that the system wants to
convert the poly to a circle, then take its center, but we don't support
poly-to-circles with radius zero.

What I have done is to add special code to allow getting the center of a
single-point polygon _without_ converting to a circle.  I think this
fixes the two cases you mentioned:

    test=> SELECT @@ polygon '((1, 1))';
     ?column?
    ----------
     (1,1)
    (1 row)

    test=> SELECT point(polygon '((1, 1))');
     point
    -------
     (1,1)
    (1 row)

but still prevents the creation of a zero-radius circle from a poly:


    test=> SELECT circle(polygon '((1, 1))');
    ERROR:  cannot convert empty polygon to circle

What is odd however, is that we allow _explicit_ creation of zero-radius
circles:

    test=> SELECT circle '<(1,1),0>';
      circle
    -----------
     <(1,1),0>
    (1 row)

so why do we prevent the conversion of single-point polygons to circles
with zero radius?  I don't know.

Patch attached.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +